$input = 'Fooo bar bazzz, фыва-олдж.';

$longest = array_reduce(preg_split('/[^a-zа-яё]+/ui', $input), function ($longest, $curr) {
    return mb_strlen($longest) < mb_strlen($curr)
        ? $curr
        : $longest;
}, '');

echo($longest); // "bazzz"

$words = explode(" ", preg_replace("/[^A-ZА-ЯЁ ]/ui", " ", $str));
$long = "";
foreach ($words as $word)
  $long = (mb_strlen($word) > mb_strlen($long)) ? $word : $long;
echo $long;